Gas Chromatography (Lecture 22) Flashcards
What is the mobile phase?
an inert gas (usually He, Ar, N₂)
the gas to be used is determined by the needs of the instrument’s detector
What are the criteria for the stationary phase?
must not react with solutes
must be thermally stable
must have low volatility
non polar solvents require a non polar stationary phase
polar solvents require a polar stationary phase
What is the most common stationary phase?
polydimethyl siloxane (non polar)
What are the different types of columns?
packed columns
capillary columns
What do packed columns contain?
They are constructed from glass, stainless steel copper or aluminium. They are filled with a particulate solid support, usually diatomaceous earth (very porous). The support acts as an active site for absorbing solute molecules.
What are the three main types of capillary columns?
walled coated tubular column
porous layer open tubular column
support coated open tubular column
What do walled coated capillary columns contain?
thin layer with stationary phase
What do porous layer capillary columns contain?
porous solid support, thin layer
What are support coated capillary columns?
They are basically the same as porous layer capillary columns but with a liquid stationary phase on a porous solid support.
What are the factors that affect sample introduction into a GC?
volatility
sample conc / purity (and appropriate ratio of analytes for resolution)
sample stability (process of injection must not cause degradation)
selection of stationary phase (must have similar polarity to sample)
How can volatility be controlled?
non-volatile samples (e.g. amino acids) can be reacted with 1-butanol to increase volatility
How can sample conc be controlled?
extract product until a sufficient amount has been collected then dilute the product to reach an appropriate conc
How can sample stability be controlled?
fragile samples that degrade easily can skip the heating phase and be directly injected into the column
How can the stationary phase be controlled?
stationary liquid phase can be altered to be more or less polar to match sample
What factors make a good detector for a GC experiment?
a low detection limit a linear response over a wide range of solute concs sensitivity for all solutes or selectivity for a specific class of solute insensitivity to a change in temperature or flow rate
What are the similarities between different types of GC detectors?
uses electrical current
What are the differences between different types of GC detectors?
(Thermal Conductivity Detector - TCD) - mobile phase exists over tungsten- rhenium wire filament - dependent on thermal conductivity of mobile phase (Flame Ionisation Detector - FID) - uses a flame to create small current (Electron Capture Detector - ECD) - considered a selective detector (Mass Spectrometer - MS) - uses a magnetic field
What is GC quantitative analysis used for?
used for analysis of environmental, clinical, pharmaceutical, biochemical, forensic, food science and petrochemical laboratory samples
What is GC quantitative analysis used for?
identification of components in a mixture
What is the accuracy of GC?
typical routine sample have an accuracy of 1-5% but as the sample conc decreases the accuracy decreases
What is the precision of GC?
- based on sample preparation and instrument
- standard deviation of 1-5% usually due to detector noise
- use of internal standards compensates for variability in injection volumes
What is the sensitivity of GC?
- based on the detector’s linear range (linear range = the range of conc over which a calibration curve is linear)
- wider the range, the greater the detection
What is the selectivity of GC?
highly selective, you can design a detector to just separate the analyte
